The invention relates to a worm-gear having an electric motor, on whose motor shaft a worm sits, an output shaft, on which a worm-wheel meshing with the worm sits, and a housing, in which the motor shaft and the output shaft are mounted.
Such worm-gears, as are used, for example, in electric steering aids, are disclosed, for example, by DE 198 11 784. Such a worm-gear is assembled in such a way that first of all the worm-wheel is fastened to the output shaft integrated in the steering column and then the worm is fitted into the motor-shaft bearing installed beforehand in the gear housing. To this end, it is necessary for the motor shaft to be located in a sliding fit in the bearing inner ring. This constitutes an unfavorable installation situation, since the inner ring and the load direction rotate relative to one another (peripheral load). Consequently every point of the inner ring is loaded during a revolution of the shaft. In principle, it is desirable, when peripheral load is present, to install the bearing ring, rotating relative to the load, via an interference fit in order to avoid relative movements between the bearing ring and the shaft, which relative movements may lead to wear of the fitting surface.
However, the hitherto conventional gear housings of worm-gears only permit the assembly sequence described above, so that an interference fit of the motor shaft in the bearing inner ring of the outer motor-shaft bearing cannot be realized.
A further problem in the hitherto known worm-gears consists in the fact that the electric motor is fully integrated in the entire unit and therefore cannot be checked separately. A check on the electric motor is only possible within the scope of the entire system after the complete dismantling of the steering aid and must be carried out by the steering-column/gear manufacturer, although the electric motor is supplied by another manufacturer. An added problem for the production sequences is that the interface between electric motor and gear is not clearly defined in the hitherto known variants.
The object of the invention is to provide a worm-gear with an electric motor in which the electric motor can be checked as a separate unit and in which the motor shaft can be connected to the inner rings of the bearings via a press fit.
According to the invention, the object is achieved by a worm-gear of the type mentioned at the beginning in which both bearing points of the motor shaft are arranged in a first part of the housing, which, when the shafts are installed, can be separated from and re-connected to a second part of the housing, in which the two bearing points of the output shaft are provided.
The divisible housing enables the electric motor together with its housing part to be removed when the worm-gear is assembled, so that, for example when used as an electric steering aid, the second part of the housing can remain assembled with the output shaft integrated in the steering column. The manufacturing sequences are also simplified, since the first part of the housing may be preassembled by the partner who is responsible for the electrical equipment of the system, for example of a vehicle, whereas the second part of the housing may be preassembled by the partner who is responsible for the mechanical components of the system. In this case, the interface between electric motor and gear can also be clearly established.
In the case of the worm-gear according to the invention, it is easily possible to remove the electric motor for maintenance purposes and to check it separately.
Due to the split housing and the omission of the hitherto rigidly predetermined assembly sequence, it is now also possible to arrange the inner rings of the bearings of the motor shaft on the latter in a press fit, a factor which is desirable when a peripheral load occurs.
In a first variant, the dividing plane of the two housing parts may lie between the worm and the electric motor, the longitudinal axis of the dividing plane intersecting the motor shaft. In this variant of a split gear housing, the housing part with the electric motor is pushed in the axial direction relative to the motor shaft into the second part of the housing and is screwed to the latter.
A further variant provides for the dividing plane of the two housing parts to lie between the longitudinal axes of the motor shaft and the output shaft.
Compared with the variant described above, the dividing plane between the longitudinal axes of the shafts permits a variable arrangement of the bearing points of the motor shafts. These bearing points may be provided at both ends of the motor shaft, or one bearing point of the motor shaft may be provided between the worm and the electric motor, and the other bearing point may be provided at one end of the motor shaft. However, in the first variant having a dividing plane intersecting the motor shaft, the bearings may only be provided between worm and electric motor and at the motor-side end of the motor shaft.
A third variant is configured in such a way that the dividing plane intersects the steering column in the axial direction, that is, the dividing plane is placed in the longitudinal axis of the steering column.
These different variants are used depending on the available construction space, the production techniques applied (in particular during the casting of the individual housing parts), and the assembly.
The bearing point at the motor-side end of the motor shaft is preferably arranged in a motor-housing cover, which can be removed from the first part of the housing.
In a preferred development of the variant having a dividing plane lying between the two longitudinal axes of the shafts, provision is made for a torque sensor to be provided on the output shaft, the measuring signal of which torque sensor can be detected by a transformer coil which is arranged in the first part of the housing and is coupled to control electronics, likewise arranged in the first part of the housing, for controlling the electric motor. The integration of transformer coil and control electronics in the first housing part constitutes a further simplification of the production sequence, in particular since the manufacturer of the motor components is usually also the manufacturer of the other electrical components and the complete preassembly may therefore be carried out at the premises of this system supplier. Only the torque sensors, usually based on the strain-gage principle, are to be provided at the output shaft.
The devices already known per se for absorbing overload moments may also be readily used in the worm-gear according to the invention. For example, the worm-wheel may be coupled to the output shaft via an overload clutch, or the motor shaft may be mounted so as to be axially displaceable against a restoring force.
The worm-gear according to the invention is preferably used in electric steering aids of vehicles (power-assisted steering), but is not restricted to this field of application.